Every one of us whose company has sold to large customers has faced this issue. It's usually some form of waiting for an order. The customer (usually - almost always - a middle manager) has told you that you are the selected "vendor." It's just a matter of a "bureaucratic" step or two and things will be great.
In the meantime, of course, you wait. And since the order is large, quite a bit depends on it. 
The inequality often causes you to hesitate about doing the right thing. Each time you talk to your "contact" you get reassurance - and after a couple of times, a little testiness. So you wait. After all, you do not want to "lose the order" because of a perceived pushy attitude on your part. So you wait.
And, more often than never, you get a very rude surprise when it's very late. "I'm REALLY sorry," says Marvin Middle Manager. "Our CEO/CFO/COO/CTO has decided to cut back on XYZ and, well, we're going to postpone this order until next Month, Quarter, Year."
So now what?
1. Realize that when Marvin Middle tells you "everything is great" that he believes it. At least he believes it 90%. Be positive and sympathetic when he explains it to you.
2. He, however, has seen things like this canceled before by his superiors. Ask him directly about the last time he's seen a cancellation and why.
3. Listen for the sound of office politics. If there are opposing camps surrounding this purchase don't assume your guy has won just because he tells you he has. (For an example just listen to the season's political rhetoric.)
4. Worry if the specs change. I recently saw an "order" coming that Marvin had specified in writing. "It just has to light up and blink," he said. No problem. A month later, there's a whole lot more than that going on. New requirements. Competing camps. Non-cash alternatives.
5. Be positive with the client of course. But plan - hard and severely - for the awful "sorry" phone call. Don't bet the business on getting the order and leave yourself no alternative but a very painful one.
6. Don't necessarily tell Marvin how badly you need the order. He can't probably do anything about it and if for some reason things get shaky he can use this fact to politely change his mind with his boss and save face at the same time. ("Yes, Mr. Big, I now agree with you. I think they are not as sound as we had hoped - financially." ) It's not very nice, but it happens more frequently than you'd hope.
7. If you have a client that gives you straight answers, doesn't hedge and always delivers, do everything you can for them -- beyond the call of duty. They are a pearl of great price.